The coaxial cables commonly used today for transmission of RF signals, such as television signals, for example, include a core containing an inner conductor and a metallic sheath surrounding the core and serving as an outer conductor. A dielectric surrounds the inner conductor and electrically insulates it from the surrounding metallic sheath. In some types of coaxial cables, air is used as the dielectric material, and electrically insulating spacers are provided at spaced locations throughout the length of the cable for holding the inner conductor coaxially within the surrounding sheath. In other known coaxial cable constructions, an expanded foam dielectric surrounds the inner conductor and fills the spaces between the inner conductor and the surrounding metallic sheath.
One important attribute of coaxial cable is its ability to propagate a signal with as little attenuation as possible. One method of measuring signal propagation is expressed as a percentage of the speed of light, commonly known as velocity of propagation (V.sub.P). Coaxial cables of the "air dielectric" type of construction have very good signal propagation characteristics, with V.sub.P values typically 90% or higher. However, these coaxial cables unfortunately have relatively limited bending characteristics and are susceptible to buckling, flattening or collapsing of the outer sheath, which adversely affect the electrical properties of the cable and render it unusable. Consequently, air dielectric type coaxial cables require very careful handling during installation to avoid such damage. Additionally, they are not recommended for use in installations requiring small radius bends or frequent reverse bends.
Coaxial cables of the "foam dielectric" type of construction, on the other hand, possess significantly better bending properties than air dielectric cables. They can be more easily installed without undue concern over buckling, flattening or collapsing of the outer sheath and they can be used in environments where air dielectric type cables are unsuitable. However, they are hampered by a somewhat lower velocity of propagation than air dielectric type cables. This reduction in V.sub.P and increase in attenuation loss is attributable to the foam dielectric.
An early foam dielectric coaxial cable used a polystyrene foam produced with a pentane blowing agent, as mentioned in U.S. Pat. No. 4,104,481 to Wilkenloh et al. While the foam dielectric provided excellent signal propagation, with velocity of propagation (V.sub.P) values of 90% and higher, the use of pentane as a blowing agent and the open cell nature of the resulting polystyrene foam were drawbacks which limited the widespread commercial use of this cable construction.
An alternative to the open cell polystyrene foam dielectrics has been to use a closed cell expanded polyolefin foam dielectric. U.S. Pat. No. 4,104,481 describes a coaxial cable with a polyolefin foam dielectric comprising polyethylene or polypropylene which is foamed using a chlorofluorocarbon blowing agent and a nucleating agent. The resulting foam dielectric possesses increased bending properties without the negative affects associated with the polystyrene/pentane systems. U.S. Pat. No. 4,472,595 to Fox et al. discloses a foam dielectric coaxial cable having enhanced handling and bending characteristics.
More recently, due to environmental concerns and governmental regulations, manufacturers of foams have discontinued the use of most chlorofluorocarbons and have turned to alternative blowing agents such as nitrogen, sulfur hexafluoride and carbon dioxide. However, the need exists to improve the signal propagation properties of foam dielectrics produced with these alternative blowing agents.